Richard SeelyThomas J. LiddyChristopher A. Rochelle
32页
查看更多>>摘要：The Nordland shale forms the caprock of the Utsira sands of the Sleipner reservoir currently used for carbon dioxide sequestration. The long-term exposure of shale rocks to supercritical carbon dioxide (scCO2), or scCO2-brine mixtures, may lead to structural and chemical changes in shale that lead to increases in permeability of inter-layers and caprocks, that may mean changes to plume migration behaviour and/or loss of seal efficiency of caprocks. A detailed study has been made of the initial pore structure of Nordland shale and the changes following accelerated treatment with scCO2. Gas sorption scanning curves have suggested that the void space of the original shale consisted of a Network (denoted 1) of micropores and smaller mesopores that is thermodynamically independent of a Network (2) of larger mesopores and macropores. This work introduces a new iodononane pre-adsorption technique to map the macroscopic (>microns) spatial distribution of micropores (<2 nm) and smaller mesopores in shales using CXT. CXT imaging of shale samples with iodononane pre-adsorbed in Network 1, or with entrapped mercury confined to only Network 2, suggested that both small- and large-sized pore networks were pervasive through the shale and associated with the continuous illite matrix phase. The feldspar and quartz grains did not form part of either network, though inter-particle macropores were found surrounding these mineral grains from CXT imaging of mercury entrapped there. Kinetic gas uptake experiments conducted on samples before and after filling Network 1 with iodononane suggested that the smaller mesopores were, despite their small size and thermodynamic independence from the macropores, still critical to mass transport, with the diffusion flux being funnelled through them. Shale surface areas obtained using the homotattic patch adsorption model were found more physically realistic than those determined via the ISO BET method since multi-linear regression of only the logarithm of the former, together with that of the Network 1 pore volume, predicted the gas-phase mass transport coefficient following treatment. This work demonstrated the need for the novel characterisation methods and data analysis presented here to properly understand the structure-transport relationship in shales exposed to scCO2.
查看更多>>摘要：This paper presents a mechanistic simulation study of one-dimensional multicomponent diffusion when the miscible injectant diffuses into a tight porous medium through the fracture/matrix interface with constant pore volume and temperature. The numerical implementation of diffusion based on the dust}' gas model uses the fugacity gradient for each component in the mixture as the driving force to the diffusive flux. The Peng-Robinson equation of state is used to model the non-ideal interactions among components in the miscible diffusive process. Phase stability analysis by minimization of the Helmholtz free energy is performed for each grid block at every time step to ensure that mixtures are single-phase fluids throughout the simulation. The main novelty of this paper lies in the diffusion model and its theoretical analysis, in which the fluid non-ideality affects the multicomponent diffusion through two pathways: the fugacity coefficients and the volume change on mixing that causes local pressures to change under ultra-low permeability in tight porous media. Previous studies based on the Maxwell-Stefan model did not consider the latter pathway, while others based on Fick's law are even more simplistic by not considering the non-ideal chemical potential. Analysis in this research showed that the Maxwell-Stefan model was inconsistent with its own assumption of no pressure gradient when non-ideal mixing was considered for tight reservoirs. The dusty gas model does not have this issue because it allows for pressure gradients to drive mass transfer by Knudsen diffusion. The non-ideal interaction of components should be properly characterized and utilized to enhance the early-time flux through the fracture/matrix interface in miscible gas injection into a tight reservoir. Case studies show that the volume change on mixing may substantially increase local pressures and the rate of mass transfer in tight reservoirs. Also, the fugacity coefficients of oil components at infinite dilution in the solvent had a major influence on the rate of diffusion. These two factors highlight the importance of properly characterizing reservoir fluids through an equation of state.
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